This invention relates to an aqueous silicone emulsion which provides an elastomeric product and to methods of preparation of such emulsions.
Emulsions of organopolysiloxanes have been known for many years and have been suggested for depositing both plastic and elastomeric silicone coatings. Such emulsion coating systems are especially desirable because of the absence of organic solvent which reduces toxicity, pollution and fire hazards. However, commercial utilization of silicone emulsion coating systems has been delayed by problems such as emulsion stability and cure system stability.
Findlay et al. in U.S. Pat. No. 3,294,725 describe one approach to a silicone emulsion coating system which employs a copolymer of diorganosiloxane units and monoorganosiloxane units. The copolymer is prepared by anionic emulsion polymerization with a surface active sulfonic acid catalyst. Findlay et al. teach that the copolymer containing pendant and terminal silanol functionality does not appear to be gelled in the emulsion, but when removed from the emulsion, the copolymer gels to a crosslinked rubber upon heating. It is taught that fillers can be added to the emulsion to improve the strength of the rubber obtained from the emulsion. For maximum stability of the emulsion according to Findlay et al, it is desirable to neutralize the acid catalyst with a base to a pH of approximately 7. The emulsion is described as quite stable.
Cekada in U.S. Pat. No. 3,355,406 describes another approach to a silicone emulsion coating system employing essentially linear hydroxyl endblocked polydialkylsiloxane. An emulsion of the polydialkylsiloxane is combined with a silsesquioxane in the form of a colloidal suspension or gel and with a crosslinking agent such as methyltrimethoxysilane. When a condensation catalyst such as dibutyltin dilaurate is added, the emulsion will form a silicone rubber film when the water is evaporated at room temperature. Cekada also suggests that a peroxide catalyst could be used instead of the condensation catalyst to obtain an emulsion which upon removal of water would deposit a silicone film which could then be cured to a rubber by applying heat. Cekada further suggests that without any catalyst a deposited film might be converted to a rubber by irradiation. Cekada shows neutral emulsions, except for one example in which the emulsion was acidic. In the present invention, a neutral emulsion is not stable on storage and does not cure to an elastomer after storage.
Huebner et al. in U.S. Pat. No. 3,706,695 describes a silicone emulsion system similar to Cekada's except that carbon black is employed instead of the silsesquioxane for reinforcement. The use of carbon black also makes the rubber product electrically conductive. Huebner et al. further teach that the emulsion containing both the organometallic condensation catalyst and the monoalkyltrialkoxysilane crosslinking agent will cure to a useful silicone rubber up to two weeks or more, but after longer storage will not cure satisfactorily. Although the cure could be regenerated by adding additional catalyst and crosslinking agent, it was recommended for purposes of storage that the emulsion be contained in two or more packages.
Nelson in Canadian Pat. No. 862,183 describes a silicone emulsion for treating fiberglass. The emulsion is similar to the Cekada or Huebner et al. systems but does not contain a reinforcing filler. It consists essentially of an emulsion of hydroxyl endblocked polydimethylsiloxane, a crosslinking silane such as an alkyltrialkoxysilane or an alkylorthosilicate, and a siloxane condensation catalyst. The emulsion must be heated to cure it on the fiberglass. Nelson also teaches that it is preferred to keep the condensation catalyst separate from the remaining emulsion by using a two bath system for treating the fiberglass. As in Cekada, Nelson shows only neutral or acidic emulsions.
Sorkin in U.S. Pat. No. 3,624,017 discloses a silicone emulsion system that rapidly cures on substrates at elevated temperatures to give a release coating. Sorkin employs an anionic emulsion polymerized copolymer similar to the copolymer employed by Findlay et al. (i.e. containing both diorganosiloxane units and monoorganosiloxane units) except that the ratio of monoorganosiloxane to diorganosiloxane units is higher. In addition, an organometallic curing catalyst such as dibutyltin dilaurate was included to effect the fast cure of the copolymer on the substrate.
A silicone emulsion system is also described in copending U.S. patent application Ser. No. 901,052, filed 4-28-79 and assigned to the same assignee as the present patent application. The emulsion system employs an anionically stabilized hydroxylated polydiorganosiloxane combined with a colloidal silica with the pH adjusted within the range 9 to 11.5. The resulting silicone emulsion does not provide a cured elastomeric product when the water is allowed to evaporate at ambient conditions immediately after the emulsion is prepared. However, if this silicone emulsion is aged at room temperature for an extended storage period such as five months, it does provide cured elastomeric products when the water is removed. It is further taught that this required storage period can be reduced to one to three days if a diorganotindicarboxylate is added to the emulsion. The presence of colloidal silica is required for a useful cure either with or without the diorganotindicarboxylate.
While this last system provides excellent elastomeric coatings suitable for a number of applications, it does require the presence of relatively expensive colloidal silica. It is one of the objects of the present invention to provide a silicone emulsion system which deposits an elastomeric product upon removal of the water under ambient conditions without the use of colloidal silica. It is also an object of the present invention to provide a silicone emulsion system in which the emulsion and the cure system are stable upon extended storgae in one package. These and other objects of the invention will be apparent to one skilled in the art upon consideration of the following description and appended claims.